Keystation 88es: Volume slider does not work and pitch bend wheel also changes volume

This started being an issue some time ago. My pitch bend wheel suddenly controls pitch and volume simultaneously. The volume changes between 1 and 63. The volume slider still does something, but it only changes the volume by about 3 steps around the volume that is commanded by the pitch bend wheel now... And it is as if this change depends on how fast I slide the slider. An example:

I turn the pitch wheel half way down, this will both set the pitch bend to -4096 and the volume to 14. If I now move the volume slider up, the value goes to 16 or 17 depending on how fast I move the slider, then it goes back to 14 when I release it. I tried resetting to factory settings by pushing + and - while turning the thing on, it didn't change.